This invention relates to an improvement to the circuitry described in pending application, Ser. No. 240,453, for "Unity Power Factor Switching Regulator" filed in the United States Patent and Trademark Office on Mar. 4, 1981 by Wally E. Rippel, and assigned to the same assignee as the present application. The disclosure of that application is incorporated herein by reference. Mr. Rippel's work was also the subject of the paper "A New Coupled Inductor Two - Phase Switching Regulator Improves Device Utilization," which applicant understands was presented at Power Con 7, March 24-27, 1980, in San Diego, Calif.
Also pertinent to the invention of the present application is U.S. Pat. No. 4,184,197 for a "DC-to-DC Switching Converter," issued to S. M. Cuk and R. D. Middlebrook, and assigned to the same assignee as the present application (hereinafter referred to as the "Cuk et al" patent). Cuk and Middlebrook's work was also the subject of the paper "Coupled-Inductor and Other Extensions of a New Optimum Topology Switching DC-To-DC Converter," which applicant understands was presented at the IEEE Industry Applications Society Annual Meeting, Oct. 2-6, 1977, at Los Angeles, Calif.
Cuk et al approached the problems of design of a dc-to-dc converter by deriving a new converter topology to achieve reduction of the number of circuit components required for the apparatus. Very generally, Cuk et al utilizes one inductor in series with the input source, another inductor in series with the load, and an energy transferring means having storage capabilities connected between the two inductances. The energy transferring means comprising a storage capacitor in series with the two inductors and a switch for alternatively connecting the junction between the first inductance and storage capacitance to ground for the source, and connecting the junction between the storage capacitor and the second capacitance to ground for the load, (e.g. FIG. 5 of Cuk et al). Cuk et al describe embodiments wherein negative inductive coupling between the input and output is achieved, and the input and/or output ripple reduced (e.g. FIGS. 8(a) and 8(1-3), and columns 6, line 45 to column 9, line 38.
A disadvantage of the circuit topology described in Cuk et al is the continued requirement for a power handling storage capacitor (see for example, capacitors 14 in FIGS. 8a and 8b). Such power capacitors are difficult to manufacture, expensive and are subject to reliability problems. The Rippel application describes a circuit which does not require the power capacitor and deals with the specific application of high speed switching regulators to battery charging circuits. One feature of the Rippel application is the use of negative inductive coupling between pairs of converters 180.degree. out of phase, to reduce peak currents through the switches while at the same time reducing the inductor size and mass.
Rippel describes particular arrangements for negatively coupling the output inductors in pairs, e.g. at page 15, line 8 to line 17, line 30, and the arrangements illustrated in FIGS. 8a and 8b. Rippel teaches that a conventional multiphase converter (shown in Rippel's FIG. 2) achieves complete cancellation of the first N-1 current harmonics at each of the three external modes, when the duty cycles of each of the N converters are equal and symmetrically staggered in time. With the negative coupling provided between pairs of inductors in what would otherwise be a conventional multiphase converter, Rippel shows that a combination of improved current form-factors and reduced total inductor masses is made possible. An improved power to weight ratio is achieved while peak switching currents and inductor losses are reduced. Rippel performs an analysis of the current harmonics flowing through each of N inductors to demonstrate that negative coupling reduces the ripple currents through the inductors, (e.g. at page 18, line 8 through page 25, line 14).
The negative coupling facet of the Rippel application is of considerable commercial interest, in that it does allow a reduction in inductor size and cost. However, the Rippel coupling technique suffers a significant disadvantage. Specifically, Rippel teaches only the coupling of one chopper to another, and consequently is limited to choppers with an even number of phases, such as two-phase (push-pull) choppers, four-phase choppers, and so on. Such choppers are seldom used; the great majority of polyphase choppers and inverters are three phase versions. To obtain three phases with the Rippel technique, one could use a six-phase (star) chopper and convert to three phases by use of an isolation transformer. This would add complexity and extra cost, with a corresponding decline in reliability.
Applicant is aware of other references which relate to the subject matter of this application. The text, "Power Semiconductor Circuits," by S. B. Dewan and A. Straughen, published by John Wiley & Sons, discusses, at pages 415-426, harmonic reduction in multiphase inverters by transformer connection. U.S. Pat. No. 3,915,048 discloses an electrical guitar circuit wherein negatively coupled inductors 18 and 19 are provided in a hum-bucking arrangement, and also in a frequency control circuit 15.
Other references typical of the art are U.S. Pat. Nos. 4,034,282 (for a chopper-stabilized power supply), 3,331,987 (for an apparatus including a bucking transformer for operating electrical discharge lamps), and 3,099,786 (for a control for an electrical power translation system). Another reference of interest is the book "Static Power Frequency Chargers," by L. Gyugji and B. R. Pelly, published by John Wiley & Sons, (e.g. at pp. 114-117).